Why MySQL can ignore time zone issues when using timestamp?

mysql> show variables like "%time_zone%";
+------------------+--------+
| Variable_name | Value |
+------------------+--------+
| system_time_zone | CST |
| time_zone | +08:00 |
+------------------+--------+
2 rows in set (0.30 sec)

mysql> desc timestamp_test;
+--------------+-----------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+--------------+-----------+------+-----+---------+----------------+
| id | int | NO | PRI | NULL | auto_increment |
| created_time | datetime | YES | | NULL | |
| created_at | timestamp | YES | | NULL | |
+--------------+-----------+------+-----+---------+----------------+
3 rows in set (0.26 sec)

mysql> insert into timestamp_test(created_time, created_at) values('2020-12-09 08:00:00', '2020-12-09 08:00:00');
Query OK, 1 row affected (0.22 sec)


mysql> select * from timestamp_test;
+----+---------------------+---------------------+
| id | created_time | created_at |
+----+---------------------+---------------------+
| 1 | 2020-12-09 08:00:00 | 2020-12-09 08:00:00 |
+----+---------------------+---------------------+
1 row in set (0.06 sec)

mysql> SET time_zone = "+00:00";
Query OK, 0 rows affected (0.03 sec)

mysql> insert into timestamp_test(created_time, created_at) values('2020-12-09 08:00:00', '2020-12-09 08:00:00');
Query OK, 1 row affected (0.03 sec)

mysql> SET time_zone = "+08:00";
Query OK, 0 rows affected (0.04 sec)

mysql> select * from timestamp_test;
+----+---------------------+---------------------+
| id | created_time | created_at |
+----+---------------------+---------------------+
| 1 | 2020-12-09 08:00:00 | 2020-12-09 08:00:00 |
| 2 | 2020-12-09 08:00:00 | 2020-12-09 16:00:00 |
+----+---------------------+---------------------+
2 rows in set (0.06 sec)

mysql> select *, unix_timestamp(created_at) from timestamp_test;
+----+---------------------+---------------------+----------------------------+
| id | created_time | created_at | unix_timestamp(created_at) |
+----+---------------------+---------------------+----------------------------+
| 1 | 2020-12-09 08:00:00 | 2020-12-09 08:00:00 | 1607472000 |
| 2 | 2020-12-09 08:00:00 | 2020-12-09 16:00:00 | 1607500800 |
+----+---------------------+---------------------+----------------------------+
2 rows in set (0.06 sec)

mysql> SET time_zone = "+00:00";
Query OK, 0 rows affected (0.09 sec)

mysql> show variables like "%time_zone%";
+------------------+--------+
| Variable_name | Value |
+------------------+--------+
| system_time_zone | CST |
| time_zone | +00:00 |
+------------------+--------+
2 rows in set (0.08 sec)

mysql> select *, unix_timestamp(created_at) from timestamp_test;
+----+---------------------+---------------------+----------------------------+
| id | created_time | created_at | unix_timestamp(created_at) |
+----+---------------------+---------------------+----------------------------+
| 1 | 2020-12-09 08:00:00 | 2020-12-09 00:00:00 | 1607472000 |
| 2 | 2020-12-09 08:00:00 | 2020-12-09 08:00:00 | 1607500800 |
+----+---------------------+---------------------+----------------------------+
2 rows in set (0.18 sec)